The space shuttle orbiter communication system transfers telemetry information about orbiter operating conditions and configurations, systems and payloads; commands to the orbiter systems to make them perform some function or configuration change; documentation from the ground that is printed out on the orbiter's teleprinter or text and graphics system; and voice communications among the flight crew members and between the fight crew and ground.

This information is transferred through hardline and radio frequency links. Hardline refers to wires that connect communicating devices, and RF refers to radio signals. RF communication takes place directly with the ground or through the TDRS.

Direct communication takes place through Air Force Satellite Control Facility remote tracking station sites, also known as space-ground link system ground stations, for military missions or through STDN ground stations for NASA missions. Direct signals from the ground to the orbiter are referred to as uplinks, and signals from the orbiter to the ground are called downlinks.

TDRS communication takes place through the White Sands Ground Terminal. These indirect signals from the TDRS to the orbiter are referred to as forward links, and the signal from the orbiter to the TDRS is called the return link. Communication with a detached payload from the orbiter is also referred to as FL, and RL is the signal from the payload to the orbiter.

The S-band FM, S-band PM, Ku-band and UHF systems are used to transfer information between the orbiter and the ground on RF signals in their frequency bands. The payload communication system is used to transfer information between the orbiter and its payloads either through hardline or RF links. The audio systems transfer voice communications throughout the orbiter, and the CCTV system is used for visually monitoring and recording activities. Communication security equipment aboard the orbiters provides the capability for encryption and decryption of operational data.

The shuttle orbiterS-band system operates in the S-band portion of the RF spectrum of 1,700 to 2,300 MHz.

S-Band Phase Modulation. The S-band PM system is used to transmit information to or receive information from the ground. This system can communicate either directly between the orbiter and ground or through the TDRS, FL or RL.

The S-band forward link (previously referred to as uplink) is phase modulated on a center carrier frequency of either 2,106.4 MHz (primary) or 2,041.9 MHz (secondary) for NASA and operates through the STDN or TDRS. The two S-band forward link frequencies would prevent interference if two orbiters were in operation at the same time, since one orbiter could select the high frequency and the other could select the low frequency.

The S-band return link (previously referred to as downlink) is phase modulated on a center carrier frequency of 2,287.5 MHz (primary) or 2,217.5 MHz (secondary) for NASA and operates through the STDN or TDRS. The two S-band return link frequencies also would prevent interference if two orbiters were in operation at the same. One orbiter could operate on the forward link high frequency of 2,106.4 MHz (primary) and a return link high frequency of 2,287.5 MHz (primary) through the STDN or TDRS, and the other could operate on the forward link low frequency of 2,041.9 MHz (secondary) and a return link low frequency of 2,217.5 MHz (secondary) through the STDN or TDRS.

The Department of Defense S-band forward link is phase modulated on a center carrier frequency of either 1,831.8 MHz (primary) or 1,775.5 MHz (secondary) from the Air Force Satellite Control Facility through its own ground stations and does not operate through the TDRS because the S-band power amplifiers are not powered in the AFSCF mode. The two S-band forward link frequencies would prevent interference if two orbiters were in operation at the same time; one orbiter could select the high frequency and the other could select the low frequency.

The Department of Defense S-band return link is phase modulated on a center carrier frequency of 2,287.5 MHz (primary) or 2,217.5 MHz (secondary) through the Air Force ground stations to the AFSCF and does not operate through the TDRS because the S-band power amplifiers are not powered in the AFSCF mode. The two S-band return link frequencies also would prevent interference if two orbiters were in operation at the same time. One orbiter could operate on the forward link high frequency of 1,831.8 MHz (primary) and a return link of 2,287.5 MHz (primary) from the AFSCF and its ground stations, and the other orbiter could operate on the forward link of 1,775.5 MHz (secondary) and a return link of 2,217.5 MHz (secondary) through the Air Force ground stations and the AFSCF.

The S-band PM forward link originates from MCC-H through the NASA STDN ground stations used for launch, lift-off, ascent or landing or through the WSGT via the TDRS system to the orbiter. The DOD S-band PM forward link originates from the AFSCF through its own ground stations to the space shuttle. NASA and the Department of Defense AFSCF have a choice of two forward link frequencies for data transfer, but both frequencies cannot be used at the same time.

The S-band PM forward link transfers a high data rate of 72 kilobits per second, consisting of two air-to-ground voice channels at 32 kbps each and one command channel at 8 kbps, two-way Doppler and two-way tone ranging.

The S-band PM forward link transfers a low data rate of 32 kbps, consisting of one air-to-ground voice channel at 24 kbps and one command channel of 8 kbps, two-way Doppler and two-way ranging. The two-way ranging does not operate through the TDRS.

The S-band PM return link can originate from one of two S-band PM transponders aboard the orbiter. Each transponder can return link on a frequency 2,287.5 MHz (primary) or 2,217.5 MHz (secondary), but not both at the same time. The S-band return link from the orbiter transmits the data through the NASA STDN ground stations used for launch, lift-off, ascent or landing or through the TDRS and TDRS system via the WSGT to the MCC-H. The DOD S-band return link from the orbiter transmits the data through the Air Force ground stations to the AFSCF.

The S-band return link high data rate of 192 kbps consists of two air-to-ground voice channels at 32 kbps each and one telemetry link of 128 kbps, two-way Doppler and two-way ranging. In the high-data-rate mode, incoming signals are directed to a preamplifier before they reach the transponder, and outgoing signals go to the preamplifier after leaving the transponder. The two-way Doppler and two-way ranging are operative only when in view of the NASA STDN ground stations at launch, lift-off, ascent or landing or in view of the Department of Defense AFSCF ground stations. The two-way ranging does not operate with the TDRSS.

The S-band return link low data rate consists of one air-to-ground voice channel at 32 kbps and one telemetry link at 64 kbps, two-way Doppler and two-way ranging. In the low-data-rate mode, incoming signals flow directly to the transponder, and outgoing signals go directly to the S-band antenna switch without amplification. As noted, the two-way Doppler and two-way ranging are used in the same manner as in the high-data-rate mode.

Four quadrant S-band PM antennas are located on the forward fuselage outer skin of the space shuttle approximately 90 degrees apart. The antennas are covered with a reusable thermal protection system. In the orbiter, on the flight deck viewed through the forward windows, the quadrant antennas are to the upper right, lower right, lower left and upper left. These antennas are the radiating elements for transmitting the S-band PM return link and for receiving the S-band PM forward link. Each quad antenna is a dual-beam unit that can look forward or aft for both transmission and reception without any physical movement. The orbiter S-band PM antenna switch assembly performs the signal switching among the two S-band transponders and any one of the four quadrant antennas. The proper antenna to be used is selected automatically, under onboard SM computer control, by real-time command from the ground or manually by the flight crew from the displays and controls on the spacecraft flight deck panel C3 with the S-band PM antenna rotary switch on panel C3 in GPC. The antenna selection is based on the computed line of sight to the NASA STDN ground station used for launch, lift-off, ascent or landing; the AFSCF ground station; or the TDRS in view. The antenna switching commands are sent to the switch assembly through the payload multiplexers/ demultiplexers.

GPC control can be inhibited to permit ground control to select an antenna other than the one currently selected by the GPC. The ground sends a real-time command to inhibit GPC control and then a second RTC to select the desired antenna. GPC control is restored by sending an RTC to enable the GPC mode. The eight antenna beam position selections available are upper left forward or aft, upper right forward or aft, lower right forward or aft, and lower left forward or aft.

Two identical S-band PM transponders in the orbiter function as a multipurpose, multimode transmitter/receiver. The transponder can simultaneously transmit/receive, transmit only or receive only. Only one transponder operates at one time; the other transponder is a redundant backup. The selected transponder transfers the forward link commands and voice to the network signal processor and receives the return link telemetry and voice from the NSP. The transponders may be cross-strapped. Transponder 1 or 2 may be used with network signal processor 1 or 2. The radio frequency sections of either transponder can be used with either preamplifier and power amplifier 1 or 2.

The dual S-band preamplifier, used in the TDRS mode for amplification, is required full-time for the FL radio frequency because of the much greater distance and, consequently, greater radio frequency loss from the TDRS to the orbiter (minimum of about 22,300 miles) than from the STDN to the orbiter (typically, slant ranges are in the low hundreds of miles). The preamplifier is not used in the AFSCF mode. One of the two units is used at a time, and the output of either unit can be cross-strapped to feed either transponder.

The selected transponders also provide a coherent turnaround of the PM forward link and PM return two-way Doppler and two-way tone ranging signals. The two-way Doppler and two-way ranging signals are operative when the orbiter is in view of the NASA STDN ground stations at launch, lift-off, ascent or landing or the AFSCF ground stations. The two-way Doppler operates through the TDRS, but the two-way ranging does not.

Two-way Doppler is used by ground stations to track the orbiter. The S-band PM forward link and PM return link are directly proportional to the forward link frequency (two-way Doppler). The S-band transponder provides a coherent turnaround of the forward link carrier frequency necessary for the two-way Doppler data. The transponder operates only when in view of the NASA STDN ground stations during launch, lift-off, ascent or landing or in view of the AFSCF ground stations. By measuring the forward link and using return link frequencies expected from the orbiter, the ground tracking station can measure the double Doppler shift that takes place and can calculate the radial velocity (range rate) of the orbiter with respect to the ground station. Because these links are PM, the S-band carrier center frequency is not affected by the modulating wave. It would be impossible to obtain valid Doppler data of the S-band carrier center frequency if it were affected by the modulating technique.

The S-band transponder also provides a subcarrier for two-way tone ranging. It is used to determine slant range from a known point to the orbiter and operates only when in view of the NASA STDN ground stations during launch, lift-off, ascent or landing or in view of the AFSCF ground stations. This capability does not operate through the TDRSS. The ground station forward-links ranging tones at 1.7 MHz and computes vehicle slant range from the time delay in receiving the return link 1.7-MHz tones to determine the orbiter's range. The orbiter azimuth is determined from the ground station antenna angles. A C-band skin-tracking mode also is provided from the ground station to track the orbiter and, again, is used only in view of the NASA STDN ground station associated with launch, lift-off, ascent or landing or in view of the AFSCF ground stations. This capability does not operate through the TDRSS.

The two onboard network signal processors receive commands (forward link) and transmit (return link) telemetry data to the selected S-band transponder. Only one NSP operates at a time; the other provides a redundant backup. The selected NSP receives either one or two analog voice channels from the onboard audio central control unit depending on whether one (in the low-data-rate mode) or both (in the high-data-rate mode) of the air-to-ground channels are being used, converts them to digital voice signals, time-division-multiplexes them with the telemetry from the pulse code modulation master unit, and sends the composite signal to the S-band PM transponder for transmission on the return link. On the forward S-band PM link, the NSP does just the reverse. It receives the composite signal from the S-band transponder and outputs it as either one or two analog voice signals to the audio central control unit. The composite forward link also has ground commands that the NSP decodes and sends to the onboard computers, which route the commands to the intended onboard systems.

S-Band Frequency Modulation. The S-band FM system cannot receive information; it is used to downlink data from seven different sources, one at a time, directly to the ground when there is a line of sight between the orbiter and STDN or Air Force ground stations. The S-band FM return link (previously referred to as downlink) can originate from two S-band transmitters aboard the space shuttle. Both transmitters are tuned to 2,250.0 MHz. The S-band FM return link can be transmitted simulta neously with the S-band PM return link to the STDN ground station or MCC-H or to the Air Force ground station at the AFSCF. It is planned that the S-band FM system be kittable for Department of Defense missions and removable for NASA missions when the TDRSS and the Ku-band system are fully operational.

During ascent, space shuttle main engine interface unit data are sent to the S-band FM system to be transmitted in real time to the ground. These data also are routed to the operations recorders for non-real-time transmission. On orbit, one of the other six services may be selected and routed through the S-band FM signal processor.

The FM signal processor is commanded to select one of these sources at a time for output to the S-band FM transmitter, which transmits it to the S-band FM return link through the STDN ground station used for launch, lift-off, ascent or landing or the DOD AFSCF ground station. The S-band FM return link transfers real-time SSME data from the engine interface units during launch at 60 kbps each or real-time video or operations recorder dumps of high- or low-data-rate telemetry at 1,024 kbps or one-track dumps of 60-kbps SSME data at 1,024 kbps or payload recorder at 25.5 kbps or 1,024 kbps or payload analog at 300 hertz or 4 MHz or payload digital data at 200 bps or 5 mbps or DOD data at 16 kbps or 256 kbps in real time or 128 kbps or 1,024 kbps of playback. The S-band FM return link does not operate through the TDRS system.

Only one of the two FM signal processors is used at a time. FM signal processor 1 interfaces with FM transmitter 1, and FM signal processor 2 interfaces with FM transmitter 2. The transmitters and processors cannot be cross-strapped.

Two hemispherical S-band FM antennas are located on the forward fuselage outer skin of the orbiter approximately 180 degrees apart. The antennas are covered with a reusable thermal protection system. In the orbiter on the flight deck, the hemispherical antennas are above the head (upper) and below the feet (lower) and radiate the S-band FM return link.

The S-band antenna switch assembly aboard the orbiter provides the signal switching among the two S-band FM transmitters and either of the two hemispherical antennas. The proper antenna is selected automatically by onboard or real-time command from the ground, computer control or manual flight crew selection from displays and controls on flight deck panel A1. In the GPC mode, the onboard SM computer selects the proper hemispherical antenna to be used whenever an S-band FM transmitter is active. The antenna selection is based on the computed line of sight to the NASA STDN ground station used for launch, lift-off, ascent or landing or the Department of Defense AFSCF ground stations.

The basic difference between the orbiter's quadrant and hemi spherical antennas is that the hemispherical antennas have a larger beamwidth while the quadrant antennas have a higher antenna gain. The hemispherical antennas are so named because there are two of them, one on top of the orbiter and one on the bottom. The quadrant antennas are so named because there are four of them, two on each side of the orbiter, one on the upper half and one on the lower half of each side, which provides nearly total coverage in all directions.

The hemi antenna switch has a port that can route RF television from the astronaut's extravehicular mobility unit to the orbiter's closed-circuit television system. An EMU TV unit can transmit television on one hemi antenna/antenna switch path to the orbiter while the S-band FM system is routing FM downlink telemetry to ground on the other hemi antenna/antenna switch path.

The Ku-band antenna aboard the space shuttle orbiter is located in the payload bay; thus, Ku-band can be used only when the orbiter is on orbit. The orbiterpayload bay doors are opened and the Ku-band antenna is deployed. The Ku-band system operates in the Ku-band portion of the radio frequency spectrum between 15,250 MHz and 17,250 MHz. The Ku-band carrier frequencies are 13.755 GHz from the TDRS to the orbiter and 15.003 GHz from the orbiter to the TDRS. Once the Ku-band antenna is deployed, the Ku-band system can be used as a communication system to transmit information to and receive information from the ground through the TDRSS. The Ku-band antenna aboard the orbiter can also be used as a radar system for target tracking objects in space, but it cannot be used simulta neously for Ku-band communications and radar operations.

When the Ku-band antenna is deployed aboard the orbiter and handover from the S-band system to the Ku-band system occurs, the orbiter onboard NSP operates with the Ku-band signal processor rather than the S-band transponder. The data stream is then directed through the K-band signal processor and Ku-band antenna to the TDRS in view, to the TDRS system WSGT and to MCC-H on the return link. The process is reversed for the forward link. If the Ku-band forward link is lost, the system reverts (fail-safe) to S-band.

The Ku-band system can handle higher quantities of data than the S-band systems. It transmits three channels of data, one of which is the same interleaved voice and telemetry processed by the S-band PM system. Two of the seven possible sources of information sent on the other two channels are payload analog, payload digital, payload interleaver bent-pipe, payload recorder, operations recorders, television and Spacelab (if flown).

The three channels of data are sent to the Ku-band signal processor to be interleaved. This signal then goes to the onboard deployed electronics assembly, which contains the transmitter, to be transmitted to the TDRS through the Ku-band antenna. The incoming signal goes through the onboard Ku-band antenna to the onboard receiver in the DEA and then through an internal electronics assembly (EA 1 is for communications and EA 2 is for radar) to the Ku-band signal processor. Voice and commands are sent to the network signal processor. A separate output from the Ku-band signal processor is directed to the text and graphics system. (As previously mentioned, since TDRS-A's Ku-band forward link is not functional, TAGS cannot operate aboard the orbiters.)

The Ku-band deployed assembly is mounted on the starboard sill longeron in the payload bay of the orbiter. It is deployed and activated after the payload bay doors are opened. The deployed assembly consists of a two-axis, gimbal-mounted, high-gain antenna; an integral gyro assembly; and a radio frequency electronics box. The gimbal motors position the Ku-band antenna, and the rate sensors determine how fast the antenna is moving.

The Ku-band deployed antenna assembly is 7 feet long and 1 foot wide when stowed in the payload bay. The parabolic antenna dish is 3 feet in diameter and is a graphite epoxy structure. The deployed antenna assembly weighs 180 pounds. The weight of the entire system is 304 pounds.

The antenna can be steered in several selectable modes under manual control by the flight crew or automatically by the SM computer. It provides the interface with the TDRS when there is a line of sight between the orbiter and TDRS.

When the shuttle reaches orbit, before the Ku-band antenna is deployed, circuit breakers on panel R15 are closed to energize thermostatically controlled heaters for the deployed electronics assembly, gimbals and antenna assembly. They provide electrical power to the Ku-band electronic elements, electronics assemblies 1 and 2, the signal processor assembly and Ku-band portions of panel A2. Actual deployment involves the controls and associated talkback displays on panel R13L. The antenna is locked in the stowed position to clear the adjacent payload bay doors and radiators when they are closed or moving.

The SPA processes and routes Ku-band FL and RL data. EA 1, containing the communication data processor and antenna control electronics, provides data to onboard displays, meters and computers. EA 2 provides control signals to configure the system for radar operations in addition to receiving and processing return radar data.

Deployment and stowage of the Ku-band deployed assembly is controlled by flight crew switches at the aft flight station. Twenty seconds are normally required to deploy or stow the DA. In the deployed position, the DA forms a 67-degree angle with the orbiter X axis. Activating the Ku electronics frees the antenna gimbals by removing the locking pins.

The antenna dish is edge-mounted on a two-axis gimbal. The alpha gimbal provides a 360-degree roll movement around the pole or axis of the gimbal. The beta gimbal provides a 162-degree pitch movement around its axis. The alpha gimbal has a stop at the lower part of its movement to prevent wraparound of the beta gimbal control cable. Since the beta gimbal has only a 162-degree movement, there is a 4-degree non-coverage zone outboard around the pole and a 32-degree non-coverage zone toward the payload bay.

As described in the S-band discussion, there are times when the Ku-band system, in view of a TDRS, is interrupted because the orbiter blocks the Ku-band antenna's view to the TDRS because orbiter attitude requirements or payloads' radiation sensitivities prohibit its use. In addition, periodically the Ku-band antenna beta cabling may require positioning to ensure that it does not become twisted in a way that could cause the antenna to bind.

As discussed under S-band, the Ku-band system's narrow beam makes it difficult for TDRS antennas to lock on to the signal. Therefore, the orbiter uses the S-band system to lock the Ku-band antenna into position first because the S-band system has a larger beamwidth. The procedure for acquiring TDRS Ku-band communication from the orbiter is described in the S-band section.

The Ku-band system forward link consists of a mode 1 and 2 through the TDRS in view. Mode 1 consists of 72-kbps data (two air-to-ground voice streams at 32 kbps each and 8 kbps of command), 128-kbps TAGS (used in place of the teleprinter) and 16-kbps synchronization. Mode 2 consists of 72-kbps operational data (two air-to-ground voice streams at 32 kbps each and 8 kbps of command).

Like the S-band system, the Ku-band antenna must be stowed before the orbiterpayload bay doors are closed in preparation for atmospheric entry. If the DA does not respond to normal stow operations, involving proper orientation and locking of the antenna, or to the stow operation itself, a direct stow switch on panel R13L is used. Setting this switch to on bypasses the normal stow control sequences and causes the DA to be driven inside the payload bay.

If neither the normal stow nor the direct stow can position the DA inside the payload bay, the DA can be jettisoned. To jettison the deployed assembly, the crew closes the circuit breakers on panel ML86B and activates the Ku ant arm and jett switches on panel A14, which causes a guillotine to cut the cables to the DA and releases a clamp holding the DA to the pivot assembly. The separation point is between the DA and deployment mechanism about 20 inches above the sill longeron. No ejective force is imparted to the DA; it is merely cut loose and the orbiter maneuvers away from it. The jettison operation takes approximately four seconds.

The space shuttle spacecraft transmits and receives through the S-band system, the TDRS in view and the TDRS system; thus, the WSGT and MCC-H are in the low-data-rate mode until the communications blackout in entry. After blackout, the space shuttle again operates in S-band through the TDRS system in the low-data-rate mode during descent to as low a view as possible until it reaches the S-band landing site ground station, which then transmits and receives in the high-data-rate mode on S-band.

The orbiterKu-band system includes a rendezvous radar that skin-tracks satellites or payloads in orbit to facilitate orbiter rendezvous with them. For large payloads that must be carried into orbit one section at a time, the orbiter will rendezvous with the payload segment currently in orbit to add on the next section. The gimbaling of the Ku-band antenna permits it to conduct a radar search for space hardware. The Ku-band system is first given the general location of the space hardware from the orbiter computer; then the antenna makes a spiral scan of the area to pinpoint the target.

Radar search for space hardware may use a wide spiral scan of up to 60 degrees. Objects may be detected by reflecting the radar beam off the surface of the target (passive mode) or by using the radar to trigger a transponder beacon on the target (active mode).

During a rendezvous operation, the radar system is used as a sensor that provides target angle and range information for updating the rendezvous navigation data. The operation is similar to using the crewman optical alignment sight or star trackers except that the radar provides target data in addition to angle data. Angle tracking maintains appropriate antenna pointing to keep the target within the antenna beam. Range tracking is accomplished by electronically measuring the time between a transmitted pulse and a return pulse from the target. The return pulse may be reflected from a passive (skin-tracked) target or cooperative target transmitter that is triggered by the radar-initiated pulse. The latter provides a longer range capability.

Angle tracking can be accomplished in two ways: computer or manual designations or automatic (auto) servo. During manually or computer-designated tracking, the antenna beam angle is positioned by services external to the Ku-band system. Computer tracking provides designated angle data based on combined target and orbiter state vector information. Manual tracking applies manually initiated rates to the antenna control system from switches at the orbiteraft flight deck station.

Automatic angle tracking applies error rates to the antenna control system from a receiving station that measures the target position relative to the antenna beam center. This closed-loop servosystem, internal to the Ku-band system, ignores external computer or manual designations.

Range tracking is always automatic, although computer-designated ranges are applied to the Ku-band system until automatic tracking is achieved. In the automatic tracking mode, the Ku-band system provides actual antenna angle, angle rate, range, and range rate data through an MDM for rendezvous and proximity operations. Data routed to panel A2 represent hard-wired azimuth, elevation, range and range rate information, which is not processed by the GPCs.

The Ku-band radar system has four steering modes for locating and tracking a target: crew-controlled; automatic; manual antenna steering, with programmed spiral search routines for angular acquisition; and range tracking (previously mentioned). In addition, there are two antenna stabilization modes.

Before any radar mode is used, the orbiter is normally maneuvered to an attitude with the minus Z axis pointing at the GPC-calculated target location. In all modes, therefore, the radar antenna is normally maneuvered around the minus Z axis.

The Ku-band system provides for antenna steering modes, each with a different combination of capabilities for acquiring and tracking a target: GPC, GPC designate, auto track and manual slew. All are mutually exclusive and crew selectable.

The GPC mode is fully automatic in all phases of target acquisition. Two GPC CRT commands are required before this mode can be initiated. One CRT command enables target position data to be routed from the GN&C to the SM antenna management program through the intercomputer data bus. The antenna management program converts the target position to antenna pointing angles and estimated orbiter-to-target range data. Another CRT command enables the antenna management program to send designated antenna pointing and range data to the Ku-band system through the payload 1 data bus and the payload forward 1 MDM.

When the GPC mode is selected, the antenna points to the commanded angles and adjusts the ranging system to the specified range. If a receive signal is detected, the automatic closed-loop system tracks the target in angles and range and provides data for the SM and GN&C computers and panel displays. If the target is not detected, the antenna is automatically commanded to search around the designated point. The antenna is inertially stabilized during the search operation.

The GPC designate mode provides the same designated pointing as the GPC mode without angle search or angle-tracking capabilities. No closed-loop angle tracking is provided. The designated angles are updated every two seconds. Range search and tracking are automatic. The antenna may be either inertially or body stabilized.

The auto tracking mode begins with manual antenna control, including a manually initiated search and inertial stabilization during the target acquisition phase. Once the target is detected, automatic angle and range tracking is initiated and manual control is inhibited until auto tracking is broken.

The manual slew mode allows manual control of antenna movement with maximum automatic range search. Once the target is detected, an automatic range track is initiated, but angles are still under manual control. No angle search is available in this mode.

The search mode involves a programmed antenna movement that causes the radar beam to describe a spiral pattern starting at the designated angles. The beam angle spirals out to a maximum of 30 degrees from the designated angle. In the GPC steering mode, the variation of the maximum angle of search from the designated point is inversely proportional to designated range. The smallest search spirals outward to a maximum of 6.2 degrees from the designated point for ranges from 145 to 300 nautical miles. At minimum ranges (zero to 8 nautical miles), the spiral search is the maximum 30 degrees from the designated point. If the antenna drive system detects but overshoots the target during a spiral search, a miniscan program is automatically initiated near the point of detection. The miniscan searches to a maximum of 9 degrees from the starting point in one minute. In the auto track steering mode, only the manually initiated main 30-degree scan is available.

In addition to angle search, the Ku-band system provides a range search. The process includes electronically varying the timing within the range system until it coincides with the time interval between the transmitted and received radar pulses. Once the intervals coincide, radar tracking is established and the range data output represents the range between the target and the orbiter. The crew can read the range and range rate values as panel and CRT parameters.

The crew is provided with two controls associated with range. Transmitter power output is automatically varied in proportion to range when a target is being tracked to keep the return signal relatively constant, regardless of range. If, however, the track is lost and the range system begins to search, the transmitter may transmit maximum power during the search. The crew can limit the maximum transmitter power by using an aft station panel control. The other control is a CRT command that limits the range search in GPC steering mode to 2,400 feet.

The Ku-band system has body and inertial antenna stabilization modes. When the body is stabilized, the antenna beam remains in a fixed relationship to the orbiter X, Y and Z axes during orbiter attitude changes. When the system is inertially stabilized, the antenna beam remains aligned to a point in inertial space, regardless of the orbiter attitude changes. Both of these modes are effective only when the radar angle-tracking system is not tracking a target. In the target angle-tracking mode, the system aligns the antenna beam to the target, and antenna movement is independent of orbiter attitude changes.

Each antenna steering mode has a specific stabilization mode except the designate mode. The designate mode selects either body or inertial stabilization on the basis of a real-time GPC command (not available to the crew). Since the designate mode provides range tracking only, inertial stabilization is effective during target tracking.

The payload communication system is used to transfer information between the orbiter and its payload or payloads. It supports hardline and radio frequency communications with a variety of payloads. The system is used to activate, check out and deactivate attached and detached payloads. The two main users of this system are NASA and the DOD. Other users include various satellite consortiums, private industry and foreign countries.

Communication with an attached payload takes place through the payload patch panel at the crew compartment flight deck aft station, which is connected to external equipment with internal systems. From this panel, the attached payload telemetry can take different paths. All standard command and telemetry signals are processed on board. Non-standard signals are sent to the ground through Ku-band. This method of transmission, referred to as bent-pipe telemetry, means that no onboard signal processing occurs before the telemetry is sent to the Ku-band system. Payload telemetry can go directly to the S-band, FM or Ku-band systems for transmission to the ground, payload analog or payload digital; to the payload recorder for later transmission; or to the payload data interleaver to be interleaved with other payload data in a selected format called the decommutator format load. The system also processes commands and tracks the status of various payload-related controls and displays.

Detached payloads communicate with the orbiter on an RF signal through the payload antenna by the payload interrogator. The main frequency carrier of the detached payload telemetry is demodulated by the PI. The telemetry is sent directly to the Ku-band system for transmission to the ground (bent-pipe telemetry) or to the payload signal processors. The PSP demodulates the subcarrier from the telemetry and sends the telemetry to the payload data interleaver to be interleaved with other payload data. Attached payload commands are routed to the PSP and then to the payload patch panel, which is also referred to as the payload station distribution panel. Detached payload commands are routed to the PSP and then to the PI for transmission through the payload antenna.

The communication interface unit replaces the PSP during DOD missions and uses space-ground-link-system-compatible payloads and ground stations to provide communication security. The CIU interfaces indirectly with the payload data interleaver through the patch panel because the CIU is treated as an attached payload accessed through a patch panel input of the payload data interleaver. The payload signal processor is hard-wired directly to the payload data interleaver.

The S-band payload antenna is located on the top of the outer skin of the orbiter's forward fuselage, just aft of the upper hemispherical antenna. The payload antenna is covered with reusable thermal protection system. This antenna is used as the radiating element for S-band transmission and reception to and from the orbiter to detached payloads through the forward link and return link.

The payload interrogator is a transmitter/receiver unit that provides full duplex RF communications between the orbiter and a detached payload. It transmits commands to and receives telemetry from NASA- or DOD-compatible payloads through the payload antenna.

Communication problems involving antenna position relative to payload position are not evident while the payload is within a half mile of the orbiter. However, to maintain good communication with the orbiter from distances of several miles, the payload must be within an 80-degree beamwidth (with reference to the minus Z axis) of the orbiter's PI antenna. The boundary of the 80-degree beamwidth is the 3-decibel point (or half-power point), which must be considered during communication with deployed payloads. This constraint is normally satisfied by the payload and retrieval process.

The payload interrogator receiver automatically acquires and tracks an unmodulated or modulated RF signal. PI telemetry is available through the operational instrumentation MDM to verify signal strength and frequency lock.

When the payload outputs a data rate that is not compatible with the payload signal processor or communication interface unit, all data received by the PI is throughput (bent-pipe) directly to the K-band signal processor through a dedicated channel that operates independently of, but parallel to, the NASA and DOD channels. Standard payload telemetry is sent to the PSP for processing before being routed to the payload data interleaver.

The payload signal processor is the command interface between the ground or flight crew and five attached/detached payload services. It is also a detached payload telemetry interface to the payload data interleaver.

The communication interface unit is used in place of the PSP whenever an SGLS-compatible payload is flown. This provides a command and telemetry path between the orbiterguidance, navigation and controlGPC and an SGLS-compatible payload or between the flight crew and an SGLS-compatible payload. The CIU passes commands and telemetry to either attached or detached payloads.

In the NASA mission configuration, the payload patch panel interfaces attached payloads to the PDI. Attached payloads are wired to specific input channels in the PPP during prelaunch activities. When the PDI is reconfigured by the flight crew, programming procedures include assigning inputs from the PPP to the desired decommutator.

In the DOD configuration, the PPP is the command and telemetry interface between the CIU and attached payloads as well as the telemetry interface for detached payloads from the CIU to the PDI.

The payload data interleaver allows the payload communication system to interface with the rest of the orbiter communication system and computers. It receives up to six different inputs from attached or detached payloads and one test input. For missions using the PSP, a maximum of five attached payloads can be accommodated on inputs 1 through 5. Input 6 is reserved for detached payload commands and telemetry using the RF link through the PSP. For missions using the CIU, all data, attached or detached, are routed through input 5. The PDI routes four of the six available inputs to the PCMMU for downlink to the SM GPC for display purposes.

The UHF system is used as a backup for the S-band PM and Ku-band voice communications primarily during extravehicular activity. For communications with the STDN ground stations, the UHF system operates in a simplex mode, which means that the orbiter flight crew can only transmit or receive, but cannot do both simultaneously. The UHF transceiver takes the voice signal from the audio central control unit and transmits it through the external UHF antenna on the bottom of the orbiterforward fuselage. The incoming UHF signal goes through the external antenna to the UHF transceiver, which sends it to the ACCU for distribution in the orbiter.

UHF transmission is controlled through the UHF mode control knob and the three two-position toggle switches on overhead panel O6 labeled xmit freq, splx pwr ampl and squelch . The xmit freq switch selects one of the two UHF frequencies, 296.8 MHz primary or 259.7 MHz secondary, for external transmission. The splx pwr ampl switch selects the UHF antenna on the external skin of the orbiter's lower forward fuselage or the airlock antenna. The UHF antenna on the lower forward fuselage is covered with reusable thermal protection system. The airlock antenna is used by the EVA astronauts, in extravehicular mobility units, to check out their transceivers before exiting the airlock; it is also used for air-to-air communications during EVA.

The squelch switch permits on or off selection of UHF squelch. A five-position rotary knob on the UHF control panel activates power to the UHF transceiver and selects any of the following modes of UHF transmission. When the knob is positioned to EVA , EVA transmissions are made on one frequency selected by the xmit freq switch, and the message is received on the other frequency. The off position removes all electrical power. When the UHF mode rotary contral knob is positioned to simplex, transmission and reception are both on the frequency selected by the xmit freq switch. Positioned to splx + g rcv, transmission and reception are the same as in simplex except that reception of the UHF guard (emergency) frequency (243.0 MHz) also is possible. In the g t/r position, transmission and reception are both on the UHF guard (emergency) frequency.

Access to transmission and reception of UHF signals is controlled by two-position toggle switches located on the bottom of the audio center panel on panel A1R at the aft station. The switches are labeled t/r for transmission/reception, off for blocking UHF signals to or from the UHF transceiver, a/g for the air-to-ground channel and a/a for air-to-air channel. All three of the UHF frequencies (296.8 MHz, 259.7 MHz and 243.0 MHz) are preset in the UHF transmitter and cannot be altered by the flight crew.

The UHF system is used for EVA operations. The EVA astronaut's UHF communication are through the orbiter UHF airlock antenna. The two existing UHF frequencies of 296.8 MHz and 259.7 MHz are used; an extra UHF of 279.0 MHz is added to the EMU backpack. The 279.0-MHz frequency can transmit or receive only among the two EVA astronauts and the orbiter, not the ground stations.

One EVA astronaut operates in mode A, transmitting data and voice to the orbiter on 259.7 MHz, transmitting voice to the other EVA astronaut on 259.7 MHz, receiving voice from the orbiter on 296.8 MHz and receiving voice from the other EVA astronaut on 279.0 MHz. The other EVA astronaut operates in mode B, transmitting data and voice to the orbiter on 279.0 MHz, transmitting voice to the other EVA astronaut on 279.0 MHz, receiving voice from the orbiter on 296.8 MHz and receiving voice from the other EVA astronaut on 259.7 MHz. The orbiter then communicates through a switch in the orbiter via the UHF EVA relay mode by retransmission over air-to-ground through its S-band system to the STDN ground station, its S-band system to the TDRS or its Ku-band system to the TDRS. As a backup procedure only when the shuttle is over a UHF ground station, the EVA astronauts, orbiter and ground can switch to the 259.7-MHz UHF, simplex. During EVA, the EVA crew members' biomedical data also are transmitted to the airlock antenna and separated from voice signals in the orbiterinstrumentation system for transmission to the ground.

The UHF system may be used after entry during the approach and landing phase of the mission. Air-to-ground voice communications take place among the space shuttle, the landing site control tower and chase planes (if used).

The audio system interfaces with the caution and warning system for reception of C/W (tone) signals; with UHF, S-band and Ku-band systems for transmission and reception of external signals (air-to-air and air-to-ground); and with the three tactical air navigation sets for receiver selection and signal monitoring. The network signal processor supplies S-band and Ku-band signals to the audio system for transmission and/or reception at various flight crew stations in the spacecraft. The attached payload (Spacelab), payload bay, launch umbilical and the operations recorders all interface electrically with the audio systems for transmissions and/or reception of signals.

The eight loops in the audio system are (1) air-to-ground 1, (2) air-to-ground 2, (3) air-to-air, (4) intercom A, (5) intercom B, (6) paging, (7) C/W and (8) TACAN. A/G 1 and A/G 2 are used to communicate with the ground through the S-band PM and Ku-band systems. In the low-data-rate mode or while the teleprinter is being used, A/G 2 is not available for voice communications. A/A is used, by convention, to communicate with the ground and with EVA astronauts through the UHF system. Intercoms A and B are used to communicate from station to station within the orbiter and Spacelab. The paging loop allows one crew member to send his voice to all active stations. The C/W loop sounds different tones for different malfunctions or emergencies. The TACAN loop, accessible only at the commander's and pilot's crew stations, is used to identify TACAN ground stations for navigation. Six audio terminal unit panels are in the orbiter at the following crew stations: commander, panel O5; pilot, panel O9; mission station, panel R10; payload station, panel L9; middeck, panel M042F; and airlock, panel AW18D.

The audio distribution system is a digital system that greatly reduces the number of wires necessary to carry electronic signals between system components. Electronic impulses can be given identifying characteristics and sorted into groups of signals. The coded impulses, called bits of information, are generated by the particular position of each switch on the various spacecraft control panels. Several bits can be reduced in number by a multiplexer to a particular identifying impulse. Bit groups from several sources are reduced so that a large number of signals can be sent along a single wire. Up to 128 bits of information can be encoded by the audio terminal unit into a serial-data word and sent along one wire to a decoder in the audio central control unit, where the bits are identified and separated by their original characteristics. Audio signals then are distributed by the ACCUs to the appropriate ADS components. There are no digital voice signals in the ADS, only digital enable signals. All ADS voice signals are analog (audio).

During launch and entry, each flight crew member wears a crew altitude protection system. The enclosed environment of the helmet lessens the severe noise levels encountered at launch and allows intelligible air-to-ground communications. For communication capability, a headset containing a microphone and earphone fits over the crew member's head, and a connector and cable interface with the headset interface unit, connected through communications cables to respective ATUs. The microphone can be positioned to suit the individual flight crew member. For emergency egress, a pull-away connection is used between the CAPS and HIU, in addition to the standard CAPS/HIU twist-on connector.

The headset interface unit has separate push-to-talk buttons for transmit and intercom modes and a volume control that determines the level of sound heard through the ear piece (microphone sound level is determined by automatic gain control circuitry within the ATU). In addition, the commander and pilot have push-to-talk switches on their rotational hand controllers for the transmit mode. Push to talk means that a push button must be depressed to allow a flight crew member to talk through the system. The headset interface unit is a portable microphone switch that is considered part of the communications carrier umbilical. It provides volume control and push-to-talk capabilities to the CCA Snoopy cap used for EVA, to the CAPS and to the wireless crew communication unit as a backup. The Snoopy cap integrates the communication carrier assembly into a skullcap. The HIU has a clip that attaches to the crew's flight suits. The unit has a three-position rocker switch and a volume control knob. The switch positions are xmit ; icom; and an unlabeled, spring-loaded-off center position. The xmit position allows access to intercom and external circuits, while the icom position is for intercom only. The volume control knob acts in series with the volume controls on the associated ATU.

The communication cables vary in configuration depending on seat location. Each seat has two 4-foot communication cables or a 14-foot length, as required. One 4-foot cable is flown as a spare. The cables connect to CCU outlets at various locations in the crew compartment. Each CCU has a specific ATU that controls communication loop configurations. CCUs are located at panels L5, R6, A11, A15 and M039M; two are located in the airlock on panel AW82D.

The CCU outlets and power switches provide electrical connection to headset and CAPS cables; an on/off toggle switch at each outlet controls electrical power to the respective cables. The commander's and pilot's stations each have three-position CCU switches labeled CCU, off and suit. The CCU position permits power flow to a WCCU or HIU. The off position blocks microphone power from a WCCU, HIU or LEH. The suit position permits power to reach the CAPS microphone. All other CCUs are two-position switches labeled CCU and off , which function the same as the commander's and pilot's switches. The CCU/EMU switches are located in the airlock below the EMU outlets on the panel labeled power/battery charger . The individual three-position switches are labeled EMU 1, EMU 2 and bus select . Each switch allows the crew to select main A or main B dc power for the EMU or to turn power on and off to the EMU. For entry communication configurations, the power controlled by the switch is used for microphone-associated circuitry only. Leaving CCU power off confines that individual to a listen-only mode, independent of ATU configuration.

A multiple headset adapter installed on the middeck ceiling is plugged into the CCU outlet on panel M039M. Its three CCU outlets allow up to three crew members seated on the middeck to share the one available outlet. When any one person connected to an MHA keys (in PTT mode) or activates the voice-operated transmitter, all three individuals' microphones will be keyed, and individuals sharing the MHA will hear each other talking only on side tone (up to 20 decibels below other loop levels). One MHA is stowed in a locker for on-orbit or backup use. Voice-activated transmission means that if a crew member's voice reaches a certain volume level (which is adjusted by a dial on each ATU), the system will transmit that crew member's voice. Hot mike means that the microphone is always keyed.

Once the shuttle reaches orbit, the crew altitude protection systems are stowed, and wireless communication units are used in place of HIUs or communication cables to allow the flight crew freedom of movement around the crew cabin.

The WCCU consists of a wall unit and a leg unit that is worn by each flight crew member in the crew compartment during orbital operations. The wall unit connects to a CCU outlet and remains attached to the crew compartment wall by Velcro until stowed for entry. Each wall and leg unit transmits on a unique pair of UHF frequencies; therefore, wall and leg units must be used together. Each set is identified by a letter on the set (e.g., A ). The wall unit is identified further by enclosing the letter in a box. Each unit is stowed with its cabling attached. The wall unit has a 23-inch cable to interface with the CCU outlet, and the leg unit has a 22-inch cable attached to a lightweight headset.

When the WCCU is unstowed, the only assembly necessary is to insert and tighten the flexible antenna in the bottom of each wall and leg unit. Since the wall unit receives power from the CCU outlet, the on/off/volume knob is not used and the battery pack is empty. The master vol control is set to full volume. All other switches are set as required; typically, the individual communication loops are used. The leg unit is stowed with battery pack installed and is attached to the crew member's leg with a wrap around elastic strap. The rotary on/off/volume knob (unlabeled) is turned clockwise past the on/off detent, and the volume is set as desired. Batteries are changed by depressing the battery pack latch push button lever (unlabeled) and sliding the battery pack off the unit. Expected use from one battery is three days (with the unit turned off during sleep periods). Sliding the new battery pack into the unit causes both the electrical connector and mechanical connector to latch.

The very lightweight headset is the interface between the leg unit and crew member. A single-strand wire headband holds the earphone against the ear and supports a thin boom holding a noise-canceling microphone near the mouth and a cable connector to the crew member's leg unit. The lightweight headset cable and connector also can interface with the HIU.

The audio central control unit, the heart of the audio system, is located in the crew compartment middeck forward avionics bay. The ACCU identifies, switches and distributes signals among the various audio distribution system components. Both digital and audio signals are received and processed by the ACCU, but the ACCU transmits only audio signals. It polls the ATUs to determine the panel configuration and connects the selected loops to the ATUs. It is also the point where the audio system is connected to the S-band PM, Ku-band and UHF systems for external communication. The selection switch for the two ACCUs is on panel C3.

The ACCU circuitry activates signals from the launch umbilical connection intercom A and B channels. Any crew station ATU then can be configured to transmit and receive intercom signals from the ground through the umbilial. (Only intercom signals are processed through the umbilical.)

Eight ATUs in the crew compartment are audio control panels used to select access by the CCU jack at each station and to control the volume of various audio signals. An ATU at each flight crew station, two in the middeck and one in the airlock control signals to headsets or CAPSs through the crew member CCU. The middeck and mission specialist crew stations have an ATU to control signals through a speaker/microphone unit. Signals to or from ATUs are processed by the ACCU. The vox, PTT, hot mic, ATU and master vol controls are controlled by circuitry within the ATU. All other knobs or switches on the face of the ATU send digital enable signals to the ACCU (except the audio function of the ATU power switch, which supplies power to the ATU circuits).

A redundancy feature of the four switches on the four ATUs allows control of a particular panel to be switched to another ATU. The left may be switched to the right commander's and pilot's ATU, and right pilot ATU control may be switched to the left commander's ATU. The commander's and pilot's control knobs are located on the respective panels. Mission specialist ATU control may be switched to the payload specialist's ATU, and airlock ATU control may be switched to the middeck and payload specialist's ATUs. The control knob for the mission specialist's ATU is located below the CRT 4 keyboard beneath the mission specialist's ATU. Control of these latter two ATUs is not reversible as commander and pilot ATU control is. Airlock CCU/ EMU 1 control is switched to the middeck CCU ATU, and airlock CCU/EMU 2 control is switched to the middeck speaker/microphone unit ATU. Both functions are switched with the single control knob in the airlock ATU. In the norm position, control of the ATU is from the panel to which the knob belongs. The other position of the knob indicates the ATU to which control can be transferred. The ATU control knob changes all ATU functions to the alternative ATU except the master volume control. This redundancy protection is used in the event of a failure or malfunction of any of the four ATUs that have an ATU control knob.

Each ATU has its own three-position power switch to control all signals to or from the ATU. The switch positions are aud/tone, aud and off. In the aud/tone position, all available functions of the ATU are armed; and transmission and receptions may be made through the ATU depending on the position of other switches on the ATU. C/W tone digital enable signals are sent to the ACCU to allow C/W audio to reach the ATU, thus the CCU or SMU. The aud position has the same functions as aud/tone except that C/W signals are blocked from the ATU. The off position shuts off power to the ATU power supply, for the ATU amplifiers. Siren ( P) and klaxon (fire) C/W signals go directly to an SMU, even with the SMU and ATU power off.

Each ATU has a two-position, spring-loaded-off paging switch that must be held in the page position to activate the circuit. When activated, the switch enables the ATU to transmit to all other ATUs, the EVA transceiver and the attached payload circuit (Spacelab). Any number of stations may use the paging circuit simultaneously, and the circuit may be used regardless of the position of the various individual channel control switches.

On all ATUs, the two air-to-ground channels, the air-to-air channel and intercom channels A and B have individual three-position control switches for selecting access to particular channels for transmission or reception. The switch positions are t/r, rcv and off . The t/r position permits transmission or reception over the selected channel. The rcv position deactivates transmission capability on the selected channel and permits only reception of signals. The off position deactivates transmission and reception on the selected channel. These control switches do not turn on any transmitter or receiver but allow access to a transmitter or receiver.

Each channel control switch has a thumbwheel volume control to adjust signal intensity on the related channel. The thumbwheels are labeled from zero (lowest volume) to nine (highest volume) and cover a range of approximately 27 decibels in 3-decibel increments. There is a volume control thumbwheel for TACAN signals on the commander's and pilot's ATUs.

The xmit/icom knob controls four combinations of external and intercom transmissions. The four knob positions are labeled PTT/hot, PTT/vox, PTT/PTT and vox/vox. In each case, the first set of initials indicates the method of external transmission activation, and the second set indicates the method of intercom transmission. In the PTT/hot position, external transmissions are made through (1) push-to-talk activation of a rotational hand controller at the commander's or pilot's station, (2) an HIU (any CCU station) ATU or (3) an SMU (middeck). Hot mike is activated, and the intercom is continuously live from the selected station ( HIU or SMU xmit must be keyed to enable external transmission). In the PTT/vox position, external transmissions are made by the transmit function of an HIU, SMU or RHC PTT; and intercom signals are voice activated. The PTT/PTT position provides access to external and intercom channels through the PTT of an RHC and HIU or an SMU, and the RHC PTT will activate any external and intercom channels selected. Vox/vox provides access to external and intercom channels and is voice activated.

Vox sensitivity regulates the loudness of the signal required for activation. The max setting requires a higher decibel level to activate the circuit than the min setting.

Master speaker volume control of all incoming signals to earphones or speakers is adjusted with the master vol knob. It acts in series with the volume control wheels for the individual channels-A/Gs 1 and 2, A/A, intercoms A and B and TACAN. Master volume knobs are located on the commander's, pilot's and airlock CCU ATUs and on the middeck SMU ATU. Two master volume knobs labeled 1 and 2 on the airlock ATU control volume to the respective CCU/EMU outlets in the airlock; the knobs are labeled from one (minimum volume) to nine (maximum volume).

The middeck ATU and the middeck speaker audio panel control operation of the SMU located at that station. In addition to the features of the other ATUs, the speaker/microphone unit ATU has a three-position power switch labeled off, spkr and spkr/mic . In the off position, no signals go through the ATU. In the spkr position, the SMU operates as a speaker only. In the spkr/mic position, the SMU can be used as either a speaker or a microphone. The SMU is located in the middeck ceiling and its power switch is located on the ATU. Signals to or from the SMU are selected on the ATU. A three-position, spring-loaded-off switch on the face of the SMU operates in conjunction with the PTT function of the ATU. The positions are xmit for access to external transmissions, icom for internal communications and the unlabeled off position for blocking outgoing PTT signals from the SMU. The xmit position sends signals over selected intercom circuits and to any external transmitters selected by the SMU ATU. The icom position excludes signals to external transmitters and allows signals to be sent over the selected intercom channel (A or B or both).

Keying the icom or xmit switch overrides the speaker, except for C/W emergency signals. In the voice-activated mode, the first signal to activate the circuit, either microphone or speaker, has priority, except for emergency C/W signals. The mic key light is a two-position, adjustable-intensity light that operates in conjunction with SMU transmissions. The intensity of the top half of the light is adjustable by the mic level knob; the brighter the light, the louder the signal. The bottom half of the light (key) is illuminated when a PTT function is selected and the circuit is keyed. Siren and klaxon C/W tones go directly to the speakers, even if the speaker power switch is off.

The audio center panel, located between the two aft viewing windows at the aft flight station, has three functions: UHF control, electrical interface capability with external vehicles and the payload bay, and operations recorders selection. All switches on the audio center panel send digital impulses to the ACCU, enabling the selected functions to communicate with Spacelab and the payload bay. Sets of on/off toggle switches labeled Spacelab and PL bay outlets electrically connect the particular function to the audio distribution system. The Spacelab subpanel has seven switches to enable the following functions: A/A, A/G 1, A/G 2, intercom A, intercom B, page and tone (C/W). The payload bay outlets subpanel has two on/off switches, one for intercom A and one for B.

Two rotary knobs labeled voice record select control various audio signals to be sent to the operational recorders through the NSP. A/G 1, A/G 2, A/A and intercom A or B audio can be sent to either recorder. Any two signals may be recorded at the same time, one on channel 1 and the other on channel 2. Either channel may be turned off. Signals to the operational recorders cannot be monitored by the flight crew.

The commander's and pilot's ATUs are the only units for controlling access to TACAN signals. There are two TACAN switches: the two-position on/off switch either allows reception or blocks incoming TACAN signals; the other three-position switch, labeled 1, 2, 3, allows selection of the particular TACAN set to be monitored. There are no transmission capabilities over the TACAN channels. TACAN is a polar coordinate system that provides distance and bearing information to a selected TACAN station. It operates in a band of frequencies from 962 to 1,213 MHz. The TACAN ground station signal identification call letters are repeated in Morse code every 40 seconds.